The present invention relates to an automatic transmission for automobiles and, more particularly, to a system for controlling feed and release of pressure oil to and from a frictional engagement element such as a clutch in the automatic transmission.
An automatic transmission for an automobile is equipped, as well known in the art, with a plurality of multi-disc clutches or brakes, which are engaged by a oil pressure so as to set gear stages by changing the power transmission lines of a gear train. In the multi-disc clutch of those frictional engagement elements, a piston made movable back and forth relative to clutch discs is fitted deep in a clutch drum having the splined clutch discs and is moved forward by feeding the oil pressure to the back thereof. As a result, the piston thrusts the clutch discs into frictional contact with the clutch discs thereby to transmit the torque.
The feed of the oil pressure to such clutch is effected via an oil passage of a shaft or boss which is arranged on the center axis of the clutch, and the release of the pressure is also effected mainly via the oil passage. The clutch is provided for connecting two revolving members and may revolve in an idle state. In this case, the residual oil is subjected to a centrifugal force so that it might be unable to go out via that oil passage. Thus, it is a current practice that the piston of the clutch is equipped with a check ball which is moved by a centrifugal force while the clutch is idly revolving to release the oil pressure from the oil passage, so that an oil release port may be opened to release the oil. The structure of this kind is disclosed, for example, in "Repair Manual of Automatic Transmission (1985-1)" issued by Toyota Motors, Co., Ltd. If, therefore, the check ball functions as expected, any unnecessary centrifugal oil pressure is not built up at the back of the piston so that the clutch can be released, as desired.
The aforementioned oil release port to be opened or closed by the check ball is formed to release the oil so that the centrifugal oil pressure may not be established, and has to be reliably closed by the check ball in case the oil pressure is fed to the back of the piston, i.e., into the cylinder. For this reliability, the oil release port is formed to have a relatively small diameter. The R.P.M. of the clutch may increase to an excessively high value, while the clutch is idly revolving, in dependence upon the depression of the accelerator pedal. In this case, the outflow, if any, of the oil of the oil release port from the cylinder is limited to build up the centrifugal oil pressure by the residual oil. As a result, the clutch has its discs thrust by the piston before most of the oil flows out of the cylinder. Accordingly, the clutch is engaged although slightly to cause the so-called "clutch drag torque". Since, in this state, the clutch discs and the clutch plates seriously slip, they are highly worn. On the other hand, the time lag till the clutch is completely released is elongated to raise another problem that the responsiveness of a shift is deteriorated to harm the shift feel of the driver.